1. Field of the Invention
The present invention relates to a suspension device for an outboard motor, a vessel propulsion apparatus including the suspension device, and a vessel to be propelled by the vessel propulsion apparatus. In particular, the present invention relates to a suspension device for an outboard motor that prevents electrolytic corrosion of a trim rod that is extendable from a trim cylinder included in a trim angle adjusting device of an outboard motor, a vessel propulsion apparatus including the suspension device, and a vessel including the vessel propulsion apparatus.
2. Description of the Related Art
A suspension device to attach an outboard motor on a hull includes a clamp bracket, a swivel bracket tiltably coupled to the clamp bracket, and a tilt cylinder device interposed between both brackets.
The clamp bracket removably fixes the suspension device to a stern of a vessel. An outboard motor is attached to the swivel bracket. The tilt cylinder device includes a tilt cylinder to perform a tilt operation and a trim cylinder to perform a trim operation.
The tilt cylinder tilts the outboard motor within a tilt range. The trim cylinder tilts the outboard motor within a trim range smaller in tilt angle than the tilt range. The tilt cylinder is used, for example, when lifting the outboard motor above a water surface during a stop or the like of the vessel. On the other hand, the trim cylinder is used, for example, when adjusting the running posture of the vessel during running of the vessel.
The trim cylinder of the tilt cylinder device includes a trim rod that is projectable and retractable from a cylinder tube of the trim cylinder. For the trim rod, which receives a thrust of the outboard motor, a strength that can withstand the thrust of the outboard motor is thus required. Therefore, the trim rod is made of, for example, an iron-based material that is excellent in strength, such as a stainless steel.
The trim rod is made of an iron-based material, whereas a cylinder block of the tilt cylinder device is made of an aluminum alloy. Aluminum has the highest ionization tendency among members of the tilt cylinder device. Thus, in order to protect the cylinder block from electrolytic corrosion, a sacrificial anode member (sacrificial electrode) called an anode (hereinafter, referred to as an anode) containing a metal such as zinc that is larger in ionization tendency than the main material such as iron and aluminum alloy is attached to the clamp bracket or the like, and the cylinder block is connected to the anode via a ground wire.
However, the trim rod is in an electrically insulated state from surrounding structural members such as a cylinder tube and a trim rod guide of the trim cylinder. Therefore, even when the cylinder tube or trim rod guide of the trim cylinder is connected to the anode by a ground wire, a sacrificial corrosion preventing effect by the anode does not reach the trim rod. Thus, when the trim rod is exposed to seawater or the like, a potential difference occurs between the trim rod and structural members around the trim rod and thus a local battery may be created. The trim rod may therefore be electrolytically corroded.
The potential difference is not only caused by a difference in the type of metal material between the trim rod and the surrounding structural members, but occurs also due to a difference in the type of metal material of the trim rod. In either case, the trim rod electrolytically corrodes due to a metal material having a large ionization tendency being ionized and eluted into seawater.
U.S. Pat. No. 8,377,266 discloses a structure in which a piston fixed to a trim rod is electrically connected to a rod guide when the rod is projected at the maximum, in order to prevent electrolytic corrosion of the trim rod. According to the structure, the trim rod is, at its maximum projection, electrically connected to the cylinder block via the piston, rod guide, and trim cylinder. It describes that the trim rod is accordingly grounded to prevent electrolytic corrosion of the trim rod.
However, during use of the outboard motor, there is little time that the trim rod is in a state of maximum projection and, thus, the trim rod is most often in a state of not being electrically connected with the rod guide. Thus, with the structure above, the effect to prevent electrolytic corrosion of the trim rod cannot be expected during use of the outboard motor. In particular, when the operating time of the outboard motor is long, the trim rod may be electrolytically corroded. As above, depending on the use of the outboard motor, the time that the trim rod is in a state other than the state of maximum projection, that is, the time that the trim rod is in a non-grounded state is longer than the time that the trim rod is in a state of maximum projection. Thus, with the above structure, the effect to prevent electrolytic corrosion of the trim rod is not always expected.